1. Field of the Invention
The invention relates generally to radio location systems. More specifically, the invention relates to a system and method for mitigating multipath interference in a radio location system including a phase difference array.
2. Description of Related Art
U.S. Patent Application Publication No. 2009/0325598 (Guigne et al.) discloses a radio location system based on a phase difference array (PDA). If P is the actual position of an object and R is a statistical measure of accuracy, then a system having an accuracy R can locate the object with an error less than R with a probability Q. The lower the value of R for a given value of Q, the higher the accuracy. The PDA radio location system measures position P with an accuracy of less than 5 cm (i.e., R<5 cm) with a probability of 68% (i.e., Q=0.68) in the absence of interference. High accuracy accords the PDA radio location system a competitive advantage over other radio location systems and allows the PDA radio location system to enable new classes of location-aware applications, such as location-based advertising at point of sale, assistive technologies, and indoor navigation systems. The PDA radio location system achieves high accuracy over a relatively short range (depending on the radio technology being used and the power of the client's transmitter), which defines the PDA radio location system as part of the near-field communications market. The only other radio technology that appears to offer accuracy comparable to the PDA radio location system is Ultra-Wideband (UWB) radio technology at an accuracy of approximately 15 cm. Other radio technologies are progressively worse in overall accuracy: Wireless Local Area Network (WLAN) technology at approximately 3-5 m, Bluetooth at approximately 2-15 m, Global Positioning System (GPS) at approximately 10 m outdoors, and Enhanced 911 (E911) technology at 50 m to 300 m.
The PDA radio location system provides high accuracy by calculating phase differences across a small compact array of antennas that is insensitive to most internal and external sources of phase noise. Although the PDA employs multiple antennas, the underlying technology of the PDA radio location system is not that of a phased array. An important difference between the PDA and traditional phased arrays is that the PDA provides a steady-state output of phase differences optimized for position calculations, whereas phased arrays are acting as spatial-temporal filters to optimize communications with position being a by-product of optimizing the signal-to-noise across the array. In the PDA radio location system, the activities of location and communications are complementary as the processing of the steady-state phase difference signals proceeds independently of the processing of the superposed communications signals. The contributions of the signal modulations to the phases cancel out when phase differences are calculated.
Multipath interference makes radio frequency location very difficult. There is a substantial body of research on mitigating multipath for traditional radio location technologies. The present invention represents the first research results on multipath mitigation for PDA radio location systems in the indoor radio environment. The main components of multipath interference are due to delay spread and angle spread. Angle spreading is due to multipath scattering in the immediate neighborhood of the transmitter, e.g., radio frequency (RF) reflections from the user, and is an issue common to all location systems. Angle spread may be considered as the time-dependent impulse response of the user. Delay spread is due to the impulse response of the indoor environment. The received signal at an antenna in the presence of delay spread is the convolution of the direct signal with the impulse response of the indoor environment. The indoor room environment's impulse response will be characterized by various length scales, e.g., the length, width and height of a given room, thickness and structure of walls, thickness and structure of floors and ceilings, the presence of obstructing objects within the room such as furniture and people, and the reflection coefficients corresponding to all of the preceding length scales. The impulse response is going to include all these effects, some of which are dynamic. In particular, the presence of people in the room will affect the overall response of the indoor environment. Thus, there are two components of delay spread: time-independent and time-dependent. The time-independent component of delay spread is due to those aspects of the room that do not change with time, i.e., infrastructure. The time-dependent components of delay spread would be due to those aspects of the room that do change with time, e.g., people moving around in the room. The orientation of the user and the user's mobile device would provide additional time-dependence of the indoor response.
Multipath signals can coherently interfere with direct path signals, and dealing with this interference is a strategic part of the PDA radio location system. The unique nature of multipath interference for PDA follows from its use of narrowband radio signals and steady-state phase differences across a geometric array of multiple sensors. In this approach, the effects of multipath are folded into the signal and multipath interference must be explicitly modeled and removed. In the Guigne et al. publication, the slope of measured phase differences between a satellite sensor and a reference sensor, called a receiver pair, versus frequency yielded a robust measure of the time difference of arrival (TDOA) of the radio signals between the receivers in that receiver pair even in the presence of multipath. The technique disclosed in the Guigne et al. publication is the first level of multipath mitigation, as it is possible to observe multipath interference as a function of frequency over multiple channels of a band of radio frequencies. The present invention expands on the mitigation technique disclosed in the Guigne et al. publication. In contrast to the PDA radio location system, the solution to the multipath interference problem for typical TDOA radio location systems requires accurate timing circuits and broadband pulses in order to separate the time of arrival (TOA) of the direct signal and isolate this signal from the later arrivals of multipath signals. This requires at least three base stations to be deployed in the same surveillance volume as served by the PDA radio location system. The PDA radio location system only requires one base station.